A.T. Robertson Commentary Revelation 5:9

"And they sing a new song, saying, Worthy art thou to take the book, and to open the seals thereof: for thou was slain, and didst purchase unto God with thy blood [men] of every tribe, and tongue, and people, and nation," — Revelation 5:9 (ASV)

They sing (αιδουσιν). Present active indicative of αιδω. Old verb, to chant with lyrical emotion (Colossians 3:16).

A new song (ωιδην καινην). Cognate accusative for οιδε (ωιδη, song) is αοιδε from αειδω, that is αιδω (the verb used), old word already used (Colossians 3:16; Ephesians 5:19), called καινην because a fresh song for new mercies (Isaiah 42:10; Psalms 33:3; 40:3, etc.), here in praise of redemption to Christ (14:3) like the new name (2:17; 3:12), the new Jerusalem (3:12; 21:2), the new heaven and the new earth (21:1), not the old song of creation (4:8,11) to God.

For thou wast slain (οτ εσφαγης). Second aorist passive indicative of σφαζω. Αγοραζω used by Paul and Peter of our purchase from sin by Christ (1 Corinthians 6:20; 1 Corinthians 7:23; Galatians 3:13; Galatians 4:5; 2 Peter 2:1; cf. 1 Peter 1:18f.).

Unto God (τω θεω). Dative case of advantage as also in verse 10.With thy blood (εν τω αιματ σου). Instrumental use of εν as in 1:5. The blood of Christ as the price of our redemption runs all through the Apocalypse. This is the reason why Christ is worthy to "take the book and open its seals." That is, he is worthy to receive adoration and worship (4:11) as the Father does.

Men of every (εκ πασης). No ανθρωπους (men) or τινας (some) before εκ in the Greek. See a like ellipsis in 11:9 with a like grouping of words for all mankind, representatives of all races and nations (7:9; 13:7; 14:6).